Such an exchange includes a time-division switching network to which a plurality of terminals are connected and to which distributed architecture control means are also connected. The control means comprises groups of identical control units, e.g. microprocessors.
Telephone service must continue even in the event of failure of one of the members of an exchange, and to ensure continued service it is conventional to install redundant units for the main functions.
It is also usual practice to connect these units by redundant links and to distribute time base signals to them in such a manner that a fault in one time base can, at worst, only affect a redundant unit.
It is common practice to group the units in security modules which are as independent as possible from one another in their links and other connections.
This kind of solution has drawbacks in an exchange of distributed architecture, particularly if the units are to be made as interchangeable as possible. This applies in particular to the control units and to the multiplex links. The purpose of making such units interchangeable is to obtain a very flexible and modular system. However, such an arrangement can lead to prohibitive multiplication of clock signal distribution means.
Preferred embodiments of the present invention remedy these drawbacks by providing a system in which the links and the clock signal distributions are organised in such a manner as to optimise the use of the redundant units and also so as to facilitate installation, maintenance and subsequent extensions.
Preferred embodiments of the invention also optimise the duration of data interchanges between units by reducing the time taken for the information to transit over the links, i.e. by making the links as short as possible.